(1):

(n.) A portion of a curved line; as, the arc of a circle or of an ellipse.

(2):

(v. i.) To form a voltaic arc, as an electrical current in a broken or disconnected circuit.

(3):

(n.) A curvature in the shape of a circular arc or an arch; as, the colored arc (the rainbow); the arc of Hadley's quadrant.

(4):

(n.) An arch.

(5):

(n.) The apparent arc described, above or below the horizon, by the sun or other celestial body. The diurnal arc is described during the daytime, the nocturnal arc during the night.